In WO 98/57436 the concept of transposition was established as a method to recreate a high frequency band from a lower frequency band of an audio signal. A substantial saving in bitrate can be obtained by using this concept in audio coding. In an HFR based audio coding system, a low bandwidth signal is presented to a core waveform coder and the higher frequencies are regenerated using transposition and additional side information of very low bitrate describing the target spectral shape at the decoder side. For low bitrates, where the bandwidth of the core coded signal is narrow, it becomes increasingly important to recreate a high band with perceptually pleasant characteristics. The harmonic transposition defined in WO 98/57436 performs well for complex musical material in a situation with low cross over frequency. The document WO 98/57436 is incorporated by reference. The principle of a harmonic transposition is that a sinusoid with frequency ω is mapped to a sinusoid with frequency Qφω where Qφ>1 is an integer defining the order of the transposition. In contrast to this, a single sideband modulation (SSB) based HFR maps a sinusoid with frequency ω to a sinusoid with frequency ω+Δω where Δω is a fixed frequency shift. Given a core signal with low bandwidth, a dissonant ringing artifact will typically result from the SSB transposition. Due to these artifacts, harmonic transposition based HFR are generally preferred over SSB based HFR.
In order to reach an improved audio quality, high quality harmonic transposition based HFR methods typically employ complex modulated filterbanks with a fine frequency resolution and a high degree of oversampling in order to reach the required audio quality. The fine frequency resolution is usually employed to avoid unwanted intermodulation distortion arising from the nonlinear treatment or processing of the different subband signals which may be regarded as sums of a plurality of sinusoids. With sufficiently narrow subbands, i.e. with a sufficiently high frequency resolution, the high quality harmonic transposition based HFR methods aim at having at most one sinusoid in each subband. As a result, intermodulation distortion caused by the nonlinear processing may be avoided. On the other hand, a high degree of oversampling in time may be beneficial in order to avoid an alias type of distortion, which may be caused by the filterbanks and the nonlinear processing. In addition, a certain degree of oversampling in frequency may be necessary to avoid pre-echoes for transient signals caused by the nonlinear processing of the subband signals.
Furthermore, harmonic transposition based HFR methods generally make use of two blocks of filterbank based processing. A first portion of the harmonic transposition based HFR typically employs an analysis/synthesis filterbank with a high frequency resolution and with time and/or frequency oversampling in order to generate a high frequency signal component from a low frequency signal component. A second portion of harmonic transposition based HFR typically employs a filterbank with a relatively coarse frequency resolution, e.g. a QMF filterbank, which is used to apply spectral side information or HFR information to the high frequency component, i.e. to perform the so-called HFR processing, in order to generate a high frequency component having the desired spectral shape. The second portion of filterbanks is also used to combine the low frequency signal component with the modified high frequency signal component in order to provide the decoded audio signal.
As a result of using a sequence of two blocks of filterbanks, and of using analysis/synthesis filterbanks with a high frequency resolution, as well as time and/or frequency oversampling, the computational complexity of harmonic transposition based HFR may be relatively high. Consequently, there is a need to provide harmonic transposition based HFR methods with reduced computational complexity, which at the same time provides good audio quality for various types of audio signals (e.g. transient and stationary audio signals).